Here are the step by steps of using those APIs in JavaScript (Node.js) in testnet3 Bitcoin network. First, create a POST request (request package is used in this example) to the first API to build transaction skeleton by passing the from address, to address and Bitcoin amount (converted to Satoshis):

Finally, send signed transaction object tmptx to Bitcoin network using second API. You can keep the final transaction object if you like. But in this example we just return the transaction hash which can be looked up in network explorer such as https://live.blockcypher.com/btc-testnet/: